There is conventionally known a developing device including a casing having an opening portion, a developing roller provided so as to be opposed to the opening portion, and a lower film for preventing toner leakage from between a lower portion of the developing roller and the casing. More specifically, in this technique, the lower film is formed into an elongated shape extending in an axial direction of the developing roller, one end of the lower film positioned upstream side of a rotational direction of the developing roller being attached to the casing by a double-stick tape, the other end thereof being brought into sliding contact with the outer peripheral surface of the developing roller as a free end.